Alloys for nitriding steel and method of nitriding steel



United States Patent 5 Claims. (Cl. 75-434 ABSTRACT OF THE DISCLOSURE Analloy for nitriding steel by introducing said alloy into the meltingsteel, the alloy comprising 40.0-90.0% by weight manganese, 0.50-10.0%by weight aluminum, 0.50-10.0% by weight nitrogen, and about and up to8%. by weight carbon.

This invention relates to a family of alloys which are introduced intomolten steel and impart to the steel a nitrogen content in predictable,given amounts.

It is well-known that during production of mild and dead soft steels,steel is charged with oxygen in an inverse ratio to its carbon content,and that simultaneous presence of oxygen and carbon leads to theformation of carbon monoxide which escapes from the molten steel andthereby generates the elrervescence or rimming action in the moltensteel.

It is also wellknown that, if oxygen is fixed by a stronger reducer thancarbon, such as silicon or aluminum, the rimming action may be sloweddown, even suppressed. Therefore, it is possible to control the rimmingaction and produce killed or semi-killed steels.

This rimming action occurs irrespective of the steel making employedincluding the new processes which use pure oxygen. However, while in theprior steel making processes the obtained steels are highly charged withnitrogen, whose excess is sometimes inconvenient, the steels obtained bythe pure oxygen processes generally contain a low amount of nitrogen,below 0.005%, even below 0.002% which is not always advantageous.

Certain dead soft steels used in the production of tin plate must besomewhat stifi and accordingly require nitrogen contents higher than0.010%

It is known to add this nitrogen through different nitrided products.For example, use of calcium cyanamide has been proposed but thiscompound presents the disadvantage, when hot in the presence of iron,alkaline carbonates and hydrgen (which elements are often present in theslag), to be partly transformed into hydrocyanic acid and cyanide andthereby be dangerous to personnel. Besides, with calcium cyanamide, thenitrogen yield is relatively low: 10% to 40%, and most irregular betweendifferent casting operations.

To obviate the disadvantages and risks of calcium cyanamide, it is alsoknown to use nitrided ferroalloys and in particular nitridedferromanganese-which effect good nitrogen yields in production ofnitrogen stainless and special steels.

However, in production of mild and dead soft steels, while nitrogenyields are more predictable or regular with nitrided ferroalloys thanwith the other nitrogen products previously proposed, the amounts ofyields are not always very high, especially in the case of rimmedsteels. This phenomenon has been explained by the fact that the oxygendissolved in steel forms an adsorption layer which slows downdissolution of the nitrogen into the iron.

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The prejudical effect of this gaseous layer might be reduced byintroduction of nitrogen combined with a strong reducing agent such asaluminum. However, due to its lightness in weight, aluminum nitride isunsuitable because it floats on top of the metal surface, is not wettedby the steel bath and remains more or less coated with slag. The resultis an extremely low and most irregular nitrogen yield in the steel.

We have found that if aluminum is alloyed with manganese, a very goodnitrogenation of this alloy is obtained whatever its carbon content.Furthermore, the dissolution speed of such nitrided alloys in moltensteel is very high and leads to most regular and predictable nitrogencontent in the steel with satisfactory yields.

Our present invention relates to:

1) Complex alloys which contain manganese, aluminum and nitrogen,

(2) A method of nitriding killed, semi-killed and rimmed steels and evenfor nitriding molten steels obtained during welding or alumino-thermicoperations.

These alloys contain:

They may also contain other components or impurities generally found inindustrial manganese alloys.

The alloys are relatively dense and they are wetted by liquid steels atnormal steel making temperatures and are most soluble therein. Theirmelting point is definitely lower than that of steel. Significantly, thepresence of aluminum renders the nitrogen dissolution in steel easierand reduces the nitrogen losses during the dissolution.

The alloys are made by known industrial processes and they are utilizedas homogeneous, crushed or sintered lumps. They are added to the steeleither in the furnace before casting, or in the runner, the feeding headsprue, or in the ladle before or during casting.

The nitrogen yield during its introduction into the steel is very highand remarkably regular for similar operating conditions.

Among the alloys, we have found particularly satisfactory thosecontaining:

Percent Mn 70 to A1 2 to 4 N2 4 t0 6 C 0 to 3 Si 0 to 2 Fe Balance Thefollowing are non-limitative examples of our in- A rimmed steel wasnitrided with this alloy to obtain a steel of the following composition:

Percent C V 0.08 to 0.10 Mn 0.40 to 0.50

N 0.010 to 0.012

For that purpose, 1.7 kg. ofthe alloy per metric ton of liquid steel,was introduced into the liquid steel. The alloy was in a sintered andcrushed form, as metallic boxes containing 500 g. of nitrogen, and wasadded together with the normal complementary additions offerromanganese.

The steel, before addition of the alloy, had the following composition:

Percent The final composition of the steel was:

Percent C 0.095

The nitrogen yield in the steel was 81%.

Example 2 A killed steel was also nitrided to obtain a steel of thefollowing composition:

Percent C 0.10to 0.13

Mn 1.40 to 1.60

Si 0.30 to 0.50

Al 0.030 to 0.040

N 0.020 to 0.025

Into the steel to be nitrided was introduced 4 kg. of the alloy of thefirst example, together with the complementary additions of refinedferromanganese, silicon and aluminum.

The molten steel, prior to addition of the-alloy, contained:

4 The final composition of the alloy was:

Percent C 0.13 Mn 1.54

Al 0.035 N 0.022

The nitrogen yield in the steel was 89%.

While we have described preferred embodiments of our invention, it maybe otherwise embodied within the scope of the appended claims.

We claim:

1. A metal alloy consisting essentially of about 40.0- 90.0% by weightmanganese, about 0.50-10.0% by weight aluminum, about 0.50-10.0% byweight nitrogen and up to 8% by weight carbon.

2. A metal alloy for nitriding steel consisting essentially ofabout-70.0-90.0% by weight manganese, about 2.04.0% by weight aluminum,about 4.06.0% by weight nitrogen, up to 3% by weight carbon, up to 2% byweight silicon and balance iron.

3. A method of nitriding steel comprising introducing into molten steelan alloy consisting essentially of about 40.0-90.0% by weight manganese,about 0.50-10.0% by weight aluminum, about 0.50-10.0% by weight nitrogenand up to 8% by weight carbon.

4. A method of nitriding steel comprising introducing into molten steelan alloy consisting essentially of 70.0- 90.0% by weight manganese,about 2.04.0% by weight aluminum, about 4.06.0% by weight nitrogen, upto 3% by weight carbon, up to 2% by weight silicon and balance iron.

5. The method of claim 3 characterized by said steel being one of amedium carbon and of a low carbon steel.

References Cited UNITED STATES PATENTS 415,832 11/1889 Cowleset al. -1342,696,433 12/1954 Tanczyn 75130.5 3,304,175 2/1967 Madsen et al. 75--134HYLAND BIZOT, Primary Examiner;

DAVID L. RECK, Examiner. RICHARD O, DEAN, Assistant Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,356,493 December 5, 1967 Francois Danis et a1.

ror appears in the above numbered pat- It is hereby certified that erthe said Letters Patent should read as ent requiring correction and thatcorrected below.

Column 2, line 25, for "20"1ead 10 Signed and sealed this 17th day ofDecember 1968.

(SEAL) Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

1. A METAL ALLOY CONSISTING ESSENTIALLY OF ABOUT 40.090.0% BY WEIGHTMANGANESE, ABOUT 0.50-10.0% BY WEIGHT ALUMINUM, ABOUT 0.50-10.0% BYWEIGHT NITROGEN AND UP TO 8% BY WEIGHT CARBON.